In a general sense, this invention relates to methods for removing material applied to or formed over a metal substrate. More specifically, it relates to methods for removing an oxide material which is disposed on a substrate, or on a coating applied over the substrate.
Metal alloys are often used in industrial environments which include extreme operating conditions. As an example, gas turbine engines are often subjected to repeated thermal cycling during operation. The standard operating temperature of turbine engines continues to be increased, to achieve improved fuel efficiency. The turbine engine components (and other industrial parts) are often formed of superalloys, which can withstand a variety of extreme operating conditions. However, they often must be covered with coatings which protect them from environmental degradation, e.g., the adverse effects of corrosion and oxidation. Current coatings used on components in gas turbine hot sections, such as blades, nozzles, combustors, and transition pieces, generally belong to one of two classes: diffusion coatings or overlay coatings.
State-of-the-art diffusion coatings are generally formed of aluminide-type alloys, such as nickel-aluminide; a noble metal-aluminide such as platinum aluminide; or nickel-platinum-aluminide. Overlay coatings typically have the composition MCrAl(X), where M is an element selected from the group consisting of Ni, Co, Fe, and combinations thereof, and X is an element selected from the group consisting of Y, Ta, Si, Hf, Ti, Zr, B, C, and combinations thereof. Diffusion coatings are formed by depositing constituent components of the coating, and reacting those components with elements from the underlying substrate, to form the coating by high temperature diffusion. In contrast, overlay coatings are generally deposited intact, without reaction with the underlying substrate.
During service, diffusion and overlay coatings on a component are often exposed to oxidative conditions. For example, coatings on turbine airfoils are typically subjected to oxidation in the hot gas path during normal operation. Under such conditions, which often include temperatures in the range of about 1400-2100° F. (about 760-1149° C.), various oxidative products (mainly thermally-grown oxide or “TGO”) are formed on the coatings. For example, aluminum oxides (especially α-aluminum oxides) often form on platinum-aluminide coatings. Aluminum oxides, chromium oxides, and various spinels often form on the MCrAl(X)-type coatings.
When turbine engine components are overhauled, the protective coatings are often removed to allow inspection and repair of the underlying substrate. Various stripping compositions have been used to remove the coatings. Usually, the oxide materials must be removed before the coatings can be treated with the stripping composition.
In past practice, oxide removal in this situation has been carried out as a separate step, prior to removal of the underlying coating. Various techniques have been used for oxide removal. For example, the oxide materials have often been removed from external sections of the turbine component by grit blasting.
As an alternative, the turbine component has sometimes been treated in an oxide-removal solution, i.e., one separate from the stripping composition used to subsequently remove the protective coating. These solutions have usually been based on strong mineral acids or strong caustics. Examples of the mineral acids are hydrochloric acid, sulfuric acid, and nitric acid. The caustic solutions usually include sodium hydroxide, potassium hydroxide, or various molten salts. Repeated treatments sometimes have to be used to remove the oxide. After removal of the oxide is completed, the substrate is then typically immersed in another solution—one that is suitable for removing the coating material itself.
These oxide removal techniques are sometimes effective, but there are often drawbacks to their use. For example, grit blasting is a labor-intensive process that is usually carried out on a piece-by-piece basis. Special care must sometimes be taken, to prevent grit-blasting damage to the substrate or any protective coating not being removed during the turbine component overhaul. Moreover, grit blasting cannot generally be used to remove oxide material from internal passage holes or cavities in the component.
Use of the oxide removal solution is advantageous in some situations, but also has drawbacks. For example, the use of two separate treatment solutions (one for removing the oxide and the other for removing the coating material) is not always desirable. A considerable amount of processing time is often involved, which can lower productivity in an industrial setting. Moreover, conventional treatment solutions which employ large quantities of strong mineral acids may emit an excessive amount of hazardous, acidic fumes. Due to environmental, health and safety concerns, such fumes must be scrubbed from ventilation exhaust systems.
Thus, new processes for removing oxide materials from coatings and/or from metal substrates would be welcome in the art. The processes should not result in the formation of an unacceptable amount of hazardous fumes. It would also be helpful if the processes were capable of removing a substantial amount of oxide material that might be located in indentations, hollow regions, or holes in the substrate, e.g., passage holes in a turbine engine substrate. Moreover, the processes should preferably be capable of being combined with other processing steps, such as a coating removal step.